Laser angioplasty is a promising method of opening arteries that are obstructed by atherosclerotic plaque. It has many potential advantages over surgery, balloon angioplasty and other forms of vascular interventions. Laser radiation may be introduced into arteries via small optical fibers, thus avoiding major surgery; the radiation can remove plaque rather than merely pushing it aside, thus, potentially reducing the high rate of restenosis that occurs with balloon angioplasty; radiation has the potential to be absorbed preferentially by plaque, thereby adding an element of specificity and safety that may not exist with mechanical atherectomy devices; and finally, the laser can remove tissue in fine increments, much smaller than what can be removed mechanically.
Early laser angioplasty systems, however, have not taken full advantage of the unique properties of laser radiation. As a result, there have been frequent perforations, dissections and other problems associated with inadvertent damage to the underlying normal artery wall. This inadvertent damage can be categorized into laser tissue interaction problems and laser delivery system problems.
Many of the laser-tissue interaction problems are now solved by careful selection of laser parameters. The massive thermal injury that occurred with continuous wave lasers can be minimized by using pulse durations that are short compared to the tissue thermal relaxation time. (Anderson RR, Parrish JA, "Selective Photothermolysis: Precise Microsurgery by Selective Absorption of Pulsed Radiation," Science 220:524-527, 1983.) Calcified plaque, which could not be removed with low to moderate intensity lasers, is now known to be readily plasma-ablated with high intensity radiation. (Prince MR, et. al., "Selective Ablation of Calcified Arterial Plaque with Laser-Induced Plasmas." IEEE J. Quantum Electronics QE23:1783-6, 1987.) Specificity for plaque (frequently called selective ablation) is achieved by choosing a wavelength where plaque absorption is much greater than normal artery absorption. (Prince MR, et. al., "Selective Laser Ablation of Atheromas Using a Flashlamp-Excited Dye Laser at 465 nm., Proc. Nat. Acad. Sci. U.S.A. 83:7064-8, 1986.) Fine, "precise" ablation is achieved at wavelengths where plaque absorption is strong or can be enhanced with exogenous chromophores.
Laser delivery problems, however, have been more difficult to resolve. The bare fibers that were used initially have been shown to have sharp edges which readily perforate arteries like a needle even when the laser is not turned on. (Sanborn et al., "Percutaneous Coronary Laser Thermal Angioplasty", Journal of American College of Cardiology 1986:8, 1437-1440) with their "hot tip" fiber, have eliminated the sharp end by covering the optical fiber with a rounded, metal cap. This rounded, bulbous cap allows the fiber to track well down arterial lumens and avoid mechanical perforation, but it forgoes many advantages of having laser radiation reach the tissue. Some investigators have melted the end of the fiber to form a ball with a similar shape as the "hot tip". This retains the desirable shape but allows all of the laser radiation to reach the tissue. Unfortunately, the laser beam spot size tends to be much smaller than the ball diameter and the tip tends to be fragile, thus creating the risk of embolizing the tip. Some of these problems have been overcome by mounting a transparent rounded window a fixed distance from the end of the fiber. Using a steel coupler, the window can be firmly attached thus avoiding the possibility of embolization. The space between the window surface and the fiber tip allows the laser beam to expand sufficiently to whatever diameter is desired. However, the window has several problems: the dead space may accumulate blood; there is a long bulky stiff end; and the extra interfaces create some loss of energy that makes it difficult to transmit enough high intensity radiation to effectively ablate material, especially calcified plaque. A fiber optic with a convex surface to change the output pattern of radiation is described by Righini in U.S. Pat. No. 4,398,790.
Ball-tipped fibers have been described in other branches of medicine also. McCaughan, in U.S. Pat. No. 4,693,556, describes a ball-tipped optical fiber for producing a spherical pattern of light. The McCaughan device is unsuitable for laser angioplasty because the energy radiates in all directions instead of forward, toward the obstructing plaque. This device is suitable for delivery of low intensity light (less than 100 watts) as the described device can not withstand the intense radiation (of the order of kilowatts to megawatts) required for pulsed, selective laser angioplasty. In addition, the McCaughan device still has an unsupported tip which can break.
It is, accordingly, a general object of this invention to provide an improved optical delivery device for laser angioplasty with a smooth, rounded, atraumatic, "lumen-seeking" tip with a large spot size, no optical interfaces within the fiber, the ability to accept intense radiation and a short stiff end.